The present invention concerns refrigeration systems, more particularly refrigeration defrost systems for defrosting a frosted evaporator.
Refrigeration systems are well known and widely used in supermarkets and warehouses to refrigerate, or maintain in a frozen state, perishable items, such as foodstuff.
Conventionally, refrigeration systems include a network of refrigeration compressors and evaporators. Refrigeration compressors mechanically compress refrigerant vapors, which are fed from the evaporators, to increase their temperature and pressure. High temperature refrigerant vapors, under high-pressure, are fed to an outdoor air-cooled refrigerant condenser whereupon air, at ambient temperature, absorbs the latent heat from the vapors, as a result the refrigerant vapors liquefy. The liquefied refrigerant is fed through expansion valves, to reduce the temperature and pressure, to the evaporators whereupon the liquefied refrigerant evaporates by absorbing heat from the surrounding foodstuff.
Since most evaporators operate at evaporating refrigerant temperatures that are lower than the freezing point of water (32xc2x0 F., 0xc2x0 C.), water vapor from ambient air freezes on the heat transfer surface of the evaporators, which creates a layer of frost on the surface. The frost layer decreases the efficiency of the heat transfer between the evaporator and the ambient air, which causes the temperature of the refrigerated space to increase above the required level. Maintaining the correct temperature of the refrigerated space is vitally important to maintain the quality of the stored food products. To do this, the evaporators must be defrosted regularly in order to reestablish their efficiency. During the defrosting period, the evaporator is out of service. It is therefore important to reduce the duration of the defrost period to avoid excessive rise of the refrigerated space temperature.
Several patents exist that have tried to solve the problem of defrosting a frosted evaporator, including:
U.S. Pat. No. 4,102,151, issued on Jul. 25, 1978, to Kramer et al, for xe2x80x9cHot Gas Defrost System with Dual Function Liquid Linexe2x80x9d.
U.S. Pat. No. 5,575,158, issued on Nov. 19, 1996, to Vogel for xe2x80x9cRefrigeration Defrost Cyclesxe2x80x9d.
U.S. Pat. No. 5,056,327, issued on Oct. 15, 1991, to Lammert for xe2x80x9cHot gas Defrost Refrigeration Systemxe2x80x9d.
U.S. Pat. No. 5,050,400, issued on Sep. 24, 1991 to Lammert for xe2x80x9cSimplified Hot Gas Defrost Refrigeration Systemxe2x80x9d.
U.S. Pat. No. 6,286,322, issued on Sep. 11, 2001 to Vogel for xe2x80x9cHot gas Refrigeration Systemxe2x80x9d.
The above systems suffer from a number of significant drawbacks such as the use of complex systems of pipes, valves, water tanks, all of which may be difficult to maintain. Disadvantageously, some of the above systems require the addition of a superheater to appropriately route the refrigerant during the defrost cycle, thereby adding to the complexity and cost of the system.
A common method for defrosting a frosted evaporator is the so-called hot refrigerant gas defrost method. Hot, high pressure refrigerant gas from a common discharge manifold or from an upper part of a refrigerant receiver, is fed backwards to the evaporator to be defrosted. The hot refrigerant gas is liquefied during its passage through the evaporator and its latent heat is used to melt the frost on the evaporator surface. The duration of the defrost period is directly proportional to the refrigerant mass flow. The higher the mass flow, the shorter the defrost period will be.
Disadvantageously, the refrigerant mass flow during a defrost cycle depends solely on the condensing pressure of the refrigeration system which, especially during the colder periods of the year, when the possibility to operate with lower condensing pressures and therefore more efficiently is readily available, is economically unacceptable.
Also, the liquid refrigerant obtained during the defrost is returned to the liquid line of the refrigeration system thus having a disruptive effect on the quality of the liquid refrigerant fed to the evaporators in normal operation, for example, so called xe2x80x9cflash gasxe2x80x9d, higher liquid temperature, and insufficient feeding of the most distant evaporators.
Thus there is a need for a refrigeration system that is simple and inexpensive to operate, and which can be used simultaneously with the normal refrigeration cycle.
The inventor has made a surprising and unexpected discovery that a single, dedicated compressor can be used to defrost a frosted evaporator in a refrigeration system. Moreover, during a defrost cycle, the single compressor operates with considerably higher suction pressure that the rest of the refrigeration compressor thus increasing efficiency and improving power consumption. Advantageously, the liquefied refrigerant is returned to the inlet of the refrigerant air cooled condenser, thus providing efficient cooling of the high pressure hot refrigerant gas before its entry into the refrigerant condenser, which increases the condenser efficiency during high ambient temperature periods of the year and reducing the condensing pressure. Another advantage is that during the cooler periods of the year, the refrigeration defrost system operates with low condensing pressures and provides efficient and rapid defrost cycle.
Also, the compressor avoids the fluctuations of the refrigeration system pressures. During a defrost cycle, a high-pressure refrigerant gas is fed to the suction of the dedicated defrost compressor thus increasing its suction pressure, mass flow and power consumption efficiency. Also during the defrost cycle, the liquid refrigerant is fed through a desuperheating expansion valve to the suction of the dedicated defrost compressor to maintain acceptable suction temperature.
In a first aspect of the present invention, there is provided a refrigeration defrost system including at least one frosted evaporator having an evaporator refrigerant vapor line and an evaporator refrigerant liquid line, said system comprising, a first compressor having a suction inlet line and a discharge outlet line each connected to a discharge manifold, said discharge outlet being connected to said evaporator refrigerant vapor line; a first pressure regulator valve disposed in a refrigerant bypass passageway between said discharge manifold and said suction inlet line, for feeding refrigerant vapor, when a defrost cycle is required, from said discharge manifold into said suction inlet line, and a first check valve in series connection with said first pressure regulator valve for stopping low pressure refrigerant vapor from said evaporator refrigerant vapor line from feeding into said suction inlet line, said refrigerant vapor being fed from said first compressor into said discharge outlet line and into said frosted evaporator through said evaporator refrigerant vapor line thereby defrosting said frosted evaporator.
In another aspect, a refrigeration defrost system, as described above, further includes a condenser having a condenser refrigerant vapor line and a condenser liquid refrigerant line, said condenser liquid refrigeration line being connected to said evaporator liquid refrigeration line, said first pressure regulator valve, during a refrigeration cycle, stops said refrigerant vapor from entering said suction inlet line, said condenser feeding liquid refrigerant into said evaporator liquid refrigerant line and said evaporator refrigerant vapor line feeding refrigerant vapor into said suction inlet line.
In another aspect, a refrigeration defrost system as described above further includes a motorized ball valve disposed in a refrigerant defrost manifold between said discharge outlet line and said evaporator, in series connection with said first pressure regulator valve, for gradually feeding said refrigerant vapor into said evaporator refrigerant vapor line.
Typically, in a refrigeration defrost system, as described above, a T-junction connects said refrigerant bypass passageway with said discharge manifold. The refrigerant bypass passageway further includes a solenoid valve and an expansion valve, in series connection between said suction inlet line and said condenser liquid refrigerant line, for feeding liquid refrigerant from said condenser liquid refrigerant line into said suction inlet line. The expansion valve is a desuperheating expansion valve.
Typically, in a refrigeration defrost system, as described above, in which said condenser further includes a liquid refrigerant return inlet line connected to said evaporator refrigerant liquid line for feeding liquefied refrigerant into said condenser during said defrost cycle. A second check valve is connected between said evaporator refrigerant liquid line and said liquid refrigerant return inlet line.
In another aspect, a refrigeration defrost system, as described above, further includes a second pressure regulator valve disposed in said discharge outlet line, said second pressure regulator valve regulating discharge outlet pressure during said defrost cycle.
Typically, a refrigeration defrost system, as described above, further includes a liquid refrigerant receiver connected between said condenser and said evaporator.
According to a second aspect of the present invention, the refrigeration defrost system further includes: first and second heat exchangers, said first heat exchanger being connected to said discharge manifold, said second heat exchanger being connected to said evaporator; a hot water tank connected to said first and second heat exchangers; and a three-way valve connected between said hot water tank and said first heat exchanger.
Typically, a three-way motorized valve is connected between said first heat exchanger and said discharge manifold, said three-way valve being closed during said defrost cycle, hot water from said hot water tank flowing into said second heat exchanger and into said frosted evaporator to defrost said frosted evaporator.
According to a third aspect of the present invention, there is provided A method of defrosting a frosted evaporator, said method comprising: feeding refrigerant vapor from a discharge manifold into a first compressor suction inlet line; feeding said refrigerant vapor from said discharge outlet line into an evaporator suction inlet line; stopping low pressure refrigerant vapor from entering said compressor suction inlet line via a first check valve, thereby defrosting said frosted evaporator.